Toxoplasmosis is a disease that results from infection with a parasite, a microbe which can only survive by living in its host's tissue, known as Toxoplasma gondii. The parasite may be acquired through eating uncooked meat or from contact with cat feces, and unborn babies may pick up the infection from their mothers. Ocular toxoplasmosis is one of the forms that the parasitic infection can take, where it involves the eyes, causing inflammation and damage. In severe cases, sight loss may result. Ocular toxoplasmosis can be treated using antibiotics and steroids but it is not possible to reverse the eye damage once it has occurred.

Worldwide, Toxoplasma gondii is the most frequent cause of inflammation within the eye. Cats are the main carrier of this parasitic disease. After entering the cat's intestines inside raw meat, Toxoplasma gondii reproduces in the gut.

A non-infectious form of the parasite, known as an oocyst, is shed from the cat inside feces, and after a few days this oocyst becomes infectious, remaining so for around a year. Humans who accidentally breathe in oocysts or eat them along with unwashed vegetables, water or soil can develop toxoplasmosis. It is also possible for humans to acquire the infection from raw meat.

In a healthy adult, toxoplasmosis typically results in a mild illness, with symptoms such as swollen glands and a rash. Ocular toxoplasmosis may develop, but it is more commonly found in association with congenital infections, which begin in the womb. Babies who pick up the disease from their mothers at the beginning of a pregnancy are usually more seriously affected. Sometimes the infection can be fatal or the baby may be born with brain abnormalities as well as being at risk of developing the eye inflammation of ocular toxoplasmosis.

Ocular toxoplasmosis causes inflammation of the retina, the part of the eye that senses light, and the choroid, a supportive layer of tissue containing blood vessels which supply the inner eye. The inflammation leads to damage and scarring, which can cause complete or partial loss of vision, or conditions such as a squint, a shrunken eye, or a clouded lens. Once scarring has occurred it is not reversible. Symptoms may include eye redness and pain, blurred, distorted vision, and the appearance of floating dark shapes.

Treatment of ocular toxoplasmosis involves a combination of different antibiotics, and in some cases steroids are also used. The aim of treatment is to prevent progression, but this is complicated by the fact that some cases of ocular toxoplasmosis arise years after the initial infection. It is possible for the disease to recur following treatment and in some cases long-term medication is needed.

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